This invention relates to inverters in general and more particularly to an inverter of the type having a control unit containing a drive unit and control device which is arranged to be ready for immediate startup.
Static inverters are sometimes used for emergency power supplies. Such emergency power supplies must be ready to take over the load at any instance. In general there are three methods for starting such static inverters. In a power supply which is truly free from interruptions, the load is supplied continuously through an inverter connected to a battery whose charging unit is connected to a supply system. If the supply system fails, the load continues to be supplied without interruption. The charging equipment is designed so that it can simultaneously deliver the full power rating of the inverter, and after a network failure, can recharge the battery. Thus, the charging unit is relatively large. The efficiency of the overall installation is poor because of the continuous double energy conversion.
If the consumer can tolerate an interruption of his power supply of the order of magnitude of the switching time of contactors, then a system in which the load is normally fed from the power system and switched to the inverter only upon failure can be used. In a standby condition the inverter operates at no-load. In such no-load operation, the firing pulses of the control unit in the inverter passes through to the control rectifiers of the power stage, the commutation circuits of which are also operating. The inverter supplies a no-load output voltage. The charging unit for the inverter must supply, in addition to the charging power after a network failure, power for the continuous idling losses of the power stage of the inverter. Since idling losses of an inverter are approximately 5% of its nominal power, the efficiency of such an installation is also poor.
If the consumer can tolerate an interval in switching over of approximately 1 second duration, systems in which the load is suppled from the network or supply system in normal operation with the inverter switched off can be used. The charging unit simply keeps the battery on a trickle charge. If the power supply fails, the inverter is switched on and after it has reached its operational state, i.e. in about 1 second, the load is connected to it. With such a system the charging unit can be of minimum size since it only need recharge the batter after a power system failure. In normal operation, the efficiency is quite favorable. However, the long interruption in resupplying the load, i.e. the 1 second start-up time is a considerable disadvantage in many applications.
Thus, it can be seen that by using the prior art systems one must either give up efficiency or tolerate unduly long interruptions. In view of this, the need for an improved system which is efficient and which at the same time provides immediate startup becomes evident.